How do I transfer my high poly sculpt to my low poly retopo mesh so I can paint it?
Your retopo attempt is looking fine. Try to Shade Smooth (right-click in Object Mode) and it should look much smoother.
You have created a low-poly mesh (step 1c, see below). If you want to animate/pose the character, you will also need to retopo the body or use a base body mesh and add the head to it.
Next, you can add a MultiRes modifier if you want and then start texturing.
A common workflow the following:

use a low-poly base mesh to form the shape of the character. This can be done in Sculpt Mode or Edit Mode. You have several options here:
a) you can use an existing base mesh and shape it in Sculpt Mode
b) OR box model it from scratch (Edit Mode)
c) OR sculpt it from scratch. The result is a high-poly mesh and you need to do a retopology to create a low-poly version from it.

optional: add a Multires Modifier to add details like wrinkles for the face, collarbones, scales, wrinkles for clothes, etc. You can transfer the details from your sculpt that you did in 1c with a Shrinkwrap Modifier.

texturing / texture baking
a) bake the Normal Map from the Multires Modifier OR bake it from the high-poly sculpt (1c) to the low-poly version. The purpose is to have a low-poly mesh but with the details of the high-poly/MultiRes version. (Low-poly + normal map == high-poly)
b) block out the base colors (Diffuse color) with a basic node setup (Ambient Occlusion and Pointiness). These nodes only work in Cycles. Baking them makes them available in Eevee and speeds up rendering
c) do a paint-over to adjust the colors. Add face color zones, make-up, etc on an extra image and mix it with the baked base colors (try MixRGB modes like Mix, Overlay, Multiply)
d) paint a Roughness map
e) optional: bake the final textures (to speed up render time and for export to game engines)